In recent years, many motor-driven vehicles such as a hybrid vehicle and an electric vehicle have been used. In many cases, such a motor-driven vehicle uses a system in which an inverter converts the direct current electric power of a battery into alternating current electric power, and a motor or a motor-generator is driven with the converted alternating current electric power. Lately, there have been many attempts to improve the driveability (speed and acceleration performance) of a motor-driven vehicle by boosting the voltage of a battery using a boost converter (voltage converter), supplying a boosted voltage to a motor, and thus increasing the range of the rotational speed and the operational torque of the motor. Lately, there has also been a motor-driven vehicle in use, in which a plurality of batteries connected in parallel are mounted so that the capacity of the batteries is increased and the motor-driven vehicle travels with a motor alone; in other words, a motor-driven vehicle which has a long EV driving range. In recent years, there has been proposed a power supply system that can switch the connection of a plurality of batteries between an in-series connection and an in-parallel connection, and enable various operation modes such as series boosting and parallel boosting by changing in various manners the on/off operation pattern of four switching elements (for example, refer to PTL 1).
The boost converter (voltage converter) adopts a boost chopper circuit that is configured to boost an input voltage by turning on and off the switching elements such as IGBTs, and thus storing electric power in a reactor or discharging electric power from the reactor. In many cases, since the temperature of the switching elements is increased due to a current passing through the switching elements, a cooler is provided in the boost converter (voltage converter) so that overheating of the elements can be prevented. However, the switching elements may be overheated depending on conditions such as the operation mode of the boost converter (voltage converter), the passing current, and the like. A reduction in the amount of current flowing via the switching elements is required so that overheating of the switching elements can be prevented, and thus input/output electric power of the battery is limited when the elements are overheated (for example, refer to PTL2). However, if input/output electric power of the battery is limited, electric power supplied to the motor is limited, thereby causing deterioration in the power performance or the driveability of the motor-driven vehicle, which is problematic.